Display screen or portion thereof with transitional graphical user interface



FIG. 1 is the first image in a sequence for a display screen or portion thereof with transitional graphical user interface showing my new design; and,

FIG. 2 is the second image thereof.

The dash-dash broken lines around the figures show a display screen or portion thereof that forms no part of the claimed design; the dash-dash broken lines within the figures show unclaimed portions of the graphical user interface. The dot-dash broken lines within the figures represent an unclaimed boundary line that forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with transitional graphical user interface, as shown and described. 